The present invention relates generally to a system for liquid level measurement for use in airplanes, boats and automobiles, for example, and more particularly to a computer coupled to one or more Hall Effect and/or resistive float transducers for measuring liquid level in a fuel tank.
Without limiting the scope of the invention, its background is described in connection to fuel tanks, more specifically aircraft fuel tanks.
Liquid level measurement in aircraft, automobiles, boats, and other vehicles has historically been measured by one or more floats connected to the arm of a variable resistor. The variable resistor, in turn, is coupled to a set of electrical leads which are brought through the wall of the fuel tank. A fuel gauge is connected to the leads into the vehicle""s electrical bus. Typically, when the float is on the bottom of the tank, the resistance sensed is low and when the float is high, the resistance sensed is high, on the order of 30 ohms. This causes the needle on the fuel gauge to deflect as the float height varies, thus indicating the quantity of fuel in the tank. For odd shaped tanks such as a flat tank, the resistance floats may be connected in series to cover this longer sloped tank.
Linearization of the output is typically performed using a calibration curve for the readout gauge, with either the radial marking locations on the dial face of an analog gauge or an electrical linearization circuit in the case of a digital gauge. In either case, an electronic interface is needed in order to display liquid levels on the vehicle""s instrumentation. The resistive float method is the fuel quantity gauging technique that is used on most, if not all, automobiles, all piston engine aircraft, and some turbine aircraft.
Another method of liquid level measurement has also been developed. An experimental method of liquid level measurement in an aircraft has been built and tested that uses a Hall Effect semiconductor device. The measurement method utilizing the Hall Effect device has been tested on a fuel system which uses a total of four float transducers, two in each tank, to measure the quantity of fuel. A linear output Hall Effect transducer is placed outside of the tank while a magnet is placed inside of a non-ferrous metal tank. The magnet is moved by the motion of a float arm as the liquid level changes in the tank. As the liquid level moves up or down, the magnet moves relative to the transducer, causing the change in transducer output voltage. This system allows liquid level measurement without any electrical connections inside the tank. This method does, however, require some electronic interface to allow the output to be used with a gauge. Additionally, there are linearity temperature effects that must be either suppressed or compensated for in the tank. A set of transducers using this technique has been built and flown with promising results.
Because both types of transducers, resistive float and Hall Effect, may potentially be available for use, a flexible electronic interface that can be used with both the resistive float and Hall Effect devices is needed. Accordingly, a means of calibrating the transducer outputs during liquid level measurement would provide numerous advantages.
The present invention provides a computer for liquid level measurement that may be interfaced with both resistive float devices and Hall Effect devices. The computer includes circuit components for calibration of electrical signals coming from one or more transducers. The electrical signals are delivered to the computer to be processed for use with readout instrumentation on-board the vehicle. The circuit components include a biasing circuit, a gain circuit, a summing circuit, and a calibration circuit.
Accordingly, disclosed in one embodiment is a computer for use in a liquid level measurement system. The computer is adapted to receive liquid level measurements from a measuring apparatus and to deliver a calibrated fuel quantity output signal to a readout instrument. The computer consists of an input interface with a first input which accepts a first liquid level measurement signal from the measuring apparatus. A first bias circuit provides an adjustable offset bias to minimize voltage errors due to input bias. Additionally, the first liquid level measurement signal is processed by a calibrating circuit which provides a calibrated fuel quantity output signal to the readout instrument. The readout instrument is connected to the computer through an output interface.
In another embodiment, a system for liquid level measurement is disclosed. The system includes a computer coupled to first and second transducers, each of which provides a corresponding first and second liquid level measurement signal to a corresponding first and second bias circuit within the computer which adjust the offset bias to minimize voltage errors due to input bias currents. Next, the first and second liquid level measurement signals are processed by first and second gain circuits, respectively, to adjust the signals so that they are balanced and matched with respect to each other. After each signal has been balanced and matched, the first and second liquid level measurement signals combine in a summing circuit so that a single summed output may be provided to a calibrating circuit. The calibrating circuit adjusts the summed output so that a calibrated fuel quantity output signal may be provided to a readout instrument through an output interface of the system.
A technical advantage of the present invention is that it can be used with both resistive float fuel transducers and Hall Effect fuel transducers.
Another technical advantage of the present invention is its ability to be located wherever it is convenient for installation on the aircraft instead of being an integral part of the transducer.